This invention relates to the assessment of drilled shaft bottom cleaning operations. The invention provides a simple, reliable and quantitative means of measuring the thickness of sediments when accessing drilled shaft base cleanliness.
Drilled shafts are high capacity deep foundation bored piles that are constructed by excavating a hole in the ground and placing fluid concrete within the excavation. Drilled shafts have several advantages over other types of deep foundations, and one of its advantages is the ability to resist high axial loads. The axial load resistance of a drilled shaft is derived for the side resistance of the concreted shafts in the surrounding soils or rock, and from end bearing resistance of the poured concrete on soil or rock. Therefore, the cleanliness of drilled shaft bases is important to achieve the desired end bearing resistance, to limit shaft settlement and to ensure that the poured concrete is not contaminated with sediments.
To achieve the desired end bearing resistance of a drilled shaft, the base of the drilled shaft must be cleaned by personnel lowered into the excavation; by mechanical means such as using a muck-bucket; or by vacuum methods. Drilled shaft base cleanliness criteria are typically specified in construction documents to limit the average thickness of sediments at the excavated shaft base to a maximum of ½ inch to 2 inches. The current state of practice to check the drilled shaft bottom cleanliness is to either: lower personnel to the bottom of the excavation to conduct downhole visual inspection; perform visual inspection using video/camera devices by personnel on the ground surface near the shaft top; or by sounding with a weighted tape lowered by personnel on the ground surface near the shaft top. These methods currently have limitations to the efficiency and effectiveness of assessing shaft bottom cleanliness, such as:
Downhole inspection by personnel lowered to the excavated drilled shaft bottom is typically avoided due to safety concerns and cannot practically be performed in wet drilled shafts.
Visual inspection using video/camera devices is relatively time consuming, uses expensive equipment and requires specialized trained personnel.
Sounding with a weighted tape is quick and easy to perform but determination of sediment thickness is subjective because it's based on a “feel” of how the weight reacts when it strikes the bottom, and is thus subject to interpretation. This can potentially lead to drilled shaft settlement and disputes.